1. Field of the Invention
The present invention relates generally to electro-optical shutter devices, and more specifically, to methods and apparatus for separating white light into packets of polarized, colored light.
2. Description of the Related Art
FIG. 1 illustrates a cross prism or cube 10 having four component prisms 32, 34, 36, 38 that fit together to give the cubic form to the cross cube 10. The cross cube 10 is an optical device that produces colored light beams 12, 14, 16 from a white light beam 18. The white light beam 18 enters the cross cube 10 through a front face 20 and the beams of red, green, and blue light 12, 14, 16 exit the cross cube 10 through the side faces 22, 24, 26. First and second color-selective layers 28, 30 give the cross cube 10 the above-described color-separating properties. The first and second layers, 28, 30 are deposited along the joints or interfaces between the four components 32, 34, 36, and 38.
The selective reflectivities of the first and second layer 28, 30 give the color-separating properties to the cross cube 10. The first layer 28 substantially reflects only red light in a first direction. The second layer 30 substantially reflects only blue light in a direction that is opposite to the first direction. Green light passes through the cross cube 10 without either a substantial reduction in intensity or a substantial deviation in direction. To use the cross cube 10 in three-color image formation, three imagers (not shown) are generally necessary, i.e. one imager for each of the separate red, green, and blue light beams 12, 14, 16. The outgoing red, green, and blue light beams 12, 14, 16 have substantially the same power per wavelength as the red, green, and blue components of the incoming white light beam 18.
Referring to FIG. 1, the source (not shown) of the white light beam 18 may be deficient in certain color or wavelength ranges. For example, metal halide lamps are deficient in red or long wavelength visible light. Such lamps output xe2x80x9cquasixe2x80x9d-white light, i.e., a white light in which one or more color components are substantially less intense than other by components. If a metal halide lamp is used with the cross cube 10 to produce the colored light beams 12, 14, 16, the red light beam 12 would be substantially less intense than the other two colored light beams 14, 16. Similarly, a sodium lamp is deficient in blue or short wavelength visible light. Therefore, using the cross cube 10 to separate quasi-white light 18 from a sodium lamp would produce the three colored light beams 12, 14, 16, but the beam of blue light 16 would be substantially less intense than the other two colored light beams 12, 14. In devices (not shown) that use the colored light beams 12, 14, 16 for color image formation, it is generally advantageous for the different colored light beams 12, 14, 16 to have photopically weighted intensities. Unfortunately, the simple cross prism 10 does not normally produce colored light beams of weighted intensity when the white light source is deficient in a particular wavelength or color range.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
In another aspect, embodiments of the invention feature an electro-optic shutter. The electro-optic shutter includes a color-selective layer, a liquid crystal device to receive light from the color-selective layer and a polarizer to receive light from the liquid crystal layer. The color-selective layer is adapted to transmit a first color, a first polarization of a second color, and a substantially orthogonal polarization of a third color. The liquid crystal device is adapted to rotate the polarization of light to a substantially orthogonal polarization when in a first state and to not substantially rotate the polarization of light when in a second state.